The plant cell wall is a complex structure consisting of different polysaccharides, the major components being cellulose, hemicelluloses and pectins. These polysaccharides may be cross-linked, or linked to lignin by phenolic acid groups such as ferulic acid. Ferulic acid may play a role in the control of cell wall growth in the plant and ferulic acid cross-linking within the cell wall is believed to restrict cell wall digestion by microorganisms (Fry et al., (1983) Planta 157: 111-123; and Borneman et al., (1990) Appl. Microbial. Biotechnol. 33: 345-351). The resistance of the plant cell wall to digestion presents significant challenges in the animal production industry. Some microorganisms are known to exhibit ferulic acid esterase activity (ferulate esterase) and thereby facilitate the breakdown of plant cell walls and fiber digestion (U.S. Pat. No. 6,143,543).
Plant cell walls contain a range of alkali-labile ester-linked phenolic acids. In particular, grass cell walls are characterized by the presence of large amounts of esterified ferulic and p-coumaric acids (mainly in their E configurations), linked to arabinoxylans at the C5 of arabinose. These are released as ferulated oligosaccharides (FAX and PAX) by cellulase treatment but in vivo provide a substrate for peroxidase-catalyzed cross-linking of cell wall polysaccharides and lignin. The high levels of these phenolic acids and their dimers have a dramatic influence on the mechanical properties, digestibility and rates of digestion of grasses by ruminants.
It has been shown that ferulic acid is the predominant p-hydroxycinnamic acid esterified to grass polysaccharide. Dehydrodiferulate dimers and cyclobutane-type dimer mixtures have been isolated from plant cell walls (Waldron, et al., (1996) Phytochemical Analysis 7:305). These mixtures are present in large amounts in grass cells. Ether linked ferulic acid-coniferyl alcohol dimers, have also been isolated from cell walls (Jacquet, et al., (1996) Polyphenol Comm. Bordeaux pp 451) establishing that ferulate esters are oxidatively co-polymerized with lignin precursors which may anchor lignins to cell wall polysaccharides. The yield of these dimers in grass cells indicates that phenolic dehydrodimer cross-linking of cell wall polysaccharides is much more extensive than was previously thought.
An enzyme system has been reported from parsley endomembranes that catalyses the ferulation of endogenous polysaccharide acceptors from feruloyl CoA, pointing to the ER/golgi as the site of polysaccharide esterification and the CoA ester as the physiological co-substrate (Meyer, et al., (1991) FEBS Lett. 290:209). Further evidence for this has been found in water-soluble extracellular polysaccharides excreted in large amounts into the medium by grass cell cultures. This material is highly esterified with ferulic and p-coumaric acid at levels similar to the cell walls of the cultured cells.
Ferulate esterase activity has been detected in several fungal species including, anaerobic gut fungi, yeasts, actinomycetes, and a few fiber-degrading ruminal bacteria, which enables them to de-esterify arabinoxylans and pectins.
Presently in livestock agriculture, while a high-forage diet is desirable, it does not currently satisfy the demands of modern animal production. Fiber digestion is a limiting factor to dairy herd milk yield and composition, and to beef production in beef operations feeding a high forage diet, and hence restricts profitability of farmers. Enhancing fiber digestion has a dual impact: 1) the animal eats more due to a reduced gut fill and therefore produces more, and 2) the animal gets more out of what it eats since the fiber is more digestible. Ultimately, these changes should increase milk yield, in dairy cows, and beef production in forage fed animals. Farmers either have to choose whether to tolerate a lower level of feed digestibility and hence productivity, or they can choose to use inoculants, forage additives or other amendments that improve the digestibility of feed.
Accordingly, farmers can treat ensiled feed or other animal feed with fiber degrading enzymes, originating mainly from molds, to improve digestibility of feed. In addition, there are several commercially available Saccharomyces cerevisiae yeast strains that when fed to cattle reportedly improve fiber digestion (Erasmus et al., (1992) J. Dairy Sci. 75: 3056-3065; and Wohlt et al., (1998) J. Dairy Sci. 81: 1345-1352). Another alternative approach to improving fiber digestion is the provision of a diet inherently possessing good digestibility characteristics. For corn silage, this may include brown midrib corn silage (Oba and Allen, (1999) J. Dairy Sci. 82: 135-142), or alternatively, corn hybrids recognized as being highly digestible. Further, new technologies incorporate fungal gene(s) responsible for the production of ferulate esterase into plant tissue for subsequent expression, resulting in improvements in fiber digestibility (WO 02/68666).
Generally, for an animal to make efficient use of the feed it consumes, the energy demands of the microorganisms in the digestive tract must be met and synchronized with the availability of plant proteins. A lack of synchrony will lead to a) proteins and other nutrients being poorly utilized in the digestive tract, b) a loss of nitrogen, in urine and feces and c) a need to feed excessive amounts of protein concentrates as supplements to the diet. The use of organisms and enzymes can improve or enhance the value of the feed animals receive and the performance of the animals. For example, WO 92/10945 discloses such a combination for use in enhancing the value of prepared silage. WO 93/13786 and WO 96/17525 relate to the enhancement of animal performance using microorganisms, while WO 93/3786 refers to a species of Lactobacillus. Further, it has been shown that Lactobacillus buchneri is suitable as a direct fed microbial to increase an animal's performance (U.S. Pat. No. 6,699,514).